Retaining piece structure

ABSTRACT

In a retaining piece interposed between the adjacent rolling balls, a concave surface of each of both side surfaces of the retaining piece where it contacts with the spherical surfaces of the balls is configured to partly contact with the spherical surface of the ball. The concave surface includes a linear contact portion to be in contact with the ball and an approximate curved surface having the radius of curvature approximate to that of the ball, which is located on the inner side of the linear contact portion. In addition, recesses which are joined to the gate portion and the ejector pin in a molding process are formed in the concave surfaces. A recess is formed at the joining portion or near a parting line. Fins formed in the molding process are capable to be put into the recess.

This is a divisional of application Ser. No. 09/756,180 filed Jan. 9,2001 now U.S. Pat. No. 6,565,947; the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a linear motion device which causes alinear motion by the utilization of the rolling of rolling elements,such as steel balls or ceramic balls. More particularly, the inventionrelates to improvements of a structure of the retaining piecesinterposed between the adjacent rolling elements and a linear motiondevice using the retaining pieces thus structured.

2. Description of the Related Art

Examples of the known linear motion device utilizing the rolling of anumber of rolling elements are a linear motion guide bearing such as alinear guide or a linear ball bearing, and various types of screwshaving balls. Those linear motion devices are widely used as importantmechanical components in many industrial machines.

As an example of this type of the device, a linear guide 10 isillustrated in plan, partly cut out, in FIG. 9. As shown, the linearguide 10 generally includes a guide rail 1, which axially extends, and aslider 2, shaped like U in cross section, which straddles the guiderail. Ball rolling grooves 3, while axially extending, are respectivelyformed in both side surfaces of the guide rail 1. A ball rolling groove5, which faces a ball rolling groove 3, is formed in the inner wall ofeach of both side portions of a slider body 2A of a slider 2. A numberof steel balls B as rolling elements are put between those opposed ballrolling grooves 3 and 5. The slider 2 is axially movable on and alongthe guide rail 1, with the assistance of the rolling of the steel ballsB. With the movement of the slider, the steel balls B, which areinterposed between the guide rail 1 and the slider 2, will move to theend of the slider body 2A of the slider 2. To continuously move theslider 2 in the axial direction, an endless circulation of those steelballs B is needed.

A linear through-hole 6 is axially formed in each side portion of theslider body 2A. The linear through-hole serves as a ball return path.End caps 7 are applied to both ends of the slider body 2A. Ballcirculation tracks 8, semicircular in cross section, are formed in thecaps so as to interconnect a path defined by the ball rolling grooves 3and 5 and the linear through-hole 6, whereby a ball endless circulationtrack 9 is formed.

A ball screw mechanism 20 is perspectively illustrated, partly cut out,in FIG. 10. As shown, a ball nut 12 is fit to an axially extendingthreaded shaft 11. A thread groove 11 a, semicircular in cross section,is formed in the outer peripheral surface of the threaded shaft 11. Athread groove 12 a, semicircular in cross section, is formed in theinner peripheral surface of the ball nut 12. The thread groove 11 a ofthe threaded shaft 11 is confronted with the semicircular thread groove12 a of the ball nut 12. A number of steel balls B as rolling elementsare put in a path defined by those thread grooves 11 a and 12 a. Thethreaded shaft 11 and the ball nut 12 are relatively moved in the axialdirection with the assistance of the rolling of the steel balls (forexample, the threaded shaft 11 is axially rotated while the ball nut 12rectilinearly moves.). With the relative movement, the steel balls Bmove while rolling in and along the spiral track defined by the threadgrooves 11 a and 12 a. To continuously move the ball nut 12, an endlesscirculation of those steel balls B is needed.

A couple of circulation holes 17 are formed in the thickness of the ballnut 12 in a state that it straddles the threaded shaft 11 and is openedto the outside of the nut. The circulation holes 17 communicativelyconnect to the spiral track while extending in the tangential directionwith respect to the path. A bail circulating track 18 is formed bycoupling the circulation holes 17 by means of a ball tube 14A shapedlike U (two ball circulating tracks 18 are used in the illustratedinstance). After the steel balls B move a distance of, for example, 1.5turn within the spiral track, those are introduced into the ballcirculating track 18 and move in an endless circulating manner.

Smoother and more stable operations of the steel balls B as rollingelements, low noise and the like are required for the linear motiondevice. To meet the requirements, a measurement has been employed inwhich a spacer ball of the reduced diameter is interposed between theadjacent load balls. Where the measure is employed, the followingproblem arises anew, however. As the result of the interposing of thespacer ball, a span between the load balls is elongated. As aninevitable consequence, the load capacity and rigidity of the linearmotion device are reduced. Therefore, a following approach isexperimentally made. Retaining pieces (also called separators) 30 areeach interposed between the adjacent steel balls B as shown in FIG. 11.By the retaining pieces 30 thus arranged, gaps in the row of the steelballs in their moving direction are eliminated and compression force isforcibly applied to the steel balls B, whereby the operability and thenoise characteristic are improved.

To further improve the operability and noise characteristic, variousproposals have been made which are constructed giving attention to theshape of the retaining piece 30 and the gaps present between theadjacent steel balls B and the retaining pieces 30.

The applicant of the present patent application proposed the followingretaining piece structures in Japanese Patent Unexamined Publication No.2000-120825:

1) In the retaining piece structure, a retaining piece has two recessedsurfaces respectively facing to the balls. Each recessed surface isshaped such that it contacts with the steel ball at its outer edge ornear outer edge. With this novel and unique technical feature, the lownoise generation is achieved, and the operability is improved bymaximizing the overlapping width of the retaining piece over which itcontacts with the steel ball. Further, the spans between the steel ballsare controlled at high precision.

2) In another structure, a retaining piece has recessed surfaces soshaped as to be in liner contact with the balls. This retaining piecestructure also achieves the low noise generation. Further, theoperability is improved since sliding resistance between the retainingpieces and the balls is small, and stable ball holding by the retainingpieces is secured.

The applicant of the present patent application proposed novel andunique retaining piece structure in Japanese Patent UnexaminedPublication No. 2000-213538. In the retaining piece structure, propergaps are provided in one ball row containing retaining pieces and balls,whereby the low noise generation and the improvement of the operabilityare achieved (those effects are high in level when the gap dimension isselected to be within 2% to 63% of the ball diameter.).

The retaining piece structure of Japanese Patent Unexamined PublicationNo. 2000-120825 is capable of satisfying the proper gap dimensiondescribed in Japanese Patent Unexamined Publication No. 2000-213538 ifit is used in normal condition and the gap dimension is within a properrange of dimension values. Under a large pre-load and a large momentload, a load acting on the retaining piece (a pressing force of the ballagainst the recessed surface of the retaining piece) is large. As aresult, the retaining piece is deformed, the ball-to-ball span isreduced, and the ball row will possibly lose its proper gap dimension.

When the linear motion device is used for a long time under hardconditions, the recessed surfaces of the retaining pieces will be wornand/or yielded. In this case, the ball-to-ball span is considerablyreduced, the proper gap dimension is lost, and the revolution of theball is abnormal. This problem remains unsolved.

An explanatory figure of the above structures is FIG. 8. In this figure,by the retaining pieces 30A arranged, gaps in the row of the steel ballsin their moving direction are eliminated and compression force isforcibly applied to the steel balls 3, whereby the operability and thenoise characteristic are improved. Further, the span between theadjacent balls is reduced to minimize the reduction of the load capacityand rigidity of the linear motion device.

Also, in the aforementioned Japanese Patent Publications, the retainingpiece 30A as the cylindrical member having the concave surfaces on bothsides to be in contact with the balls B, B is made of synthetic resinand manufactured by injection molding as shown in FIG. 8. In thisFigure, a gate portion G is provided on the outer peripheral surfacethereof.

In the case of the retaining pieces 30A manufactured in a state that thegate portion G is provided on the outer peripheral surface thereof, thereduction of the load capacity and rigidity may be lessened by reducingthe thickness of the retaining pieces 30A as small as possible. However,a gate-cutting step is inevitably needed after the injection moldingprocess. Therefore, the cost to manufacture is increased by its cost.Further, the gate remainder after the gate cutting and the parting lineof the molded product take the form of fins. As a result, theoperability and the noise characteristic of the retaining pieces willpossibly be deteriorated.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem, and hasan first object to provide a retaining piece structure and a linearmotion device using the retaining pieces thus structured in which evenwhen a load acting on the retaining pieces is excessive and the recessedsurfaces of the retaining pieces are worn and/or yielded, a variation ofthe ball-to-ball span is minimized, and hence excellent improvements ofthe operability and the noise characteristic are maintained for a longtime even under various use conditions.

In addition, the present invention has been made to solve the aboveproblems and has a second object to provide a structure of a retainingpiece and a linear motion device using the retaining pieces thusstructured in which improvement is made about the gate position and thelike in injection molding retaining pieces, whereby the operability andthe noise characteristic are reliably improved at low cost.

To achieve the above first object, there is provided a structure of aretaining piece which is interposed each between the adjacent balls of arow of rolling balls, and concave surfaces of both side surfaces of saidretaining piece as viewed in the axial direction are in contact with thespherical surfaces of the balls. The retaining piece structure ischaracterized in that each said concave surface is configured such thatas a pressing force acting on said concave surfaces increases, each saidconcave surface increases a contact area thereof with the ball.

According to a second aspect of the invention, there is provided astructure of a retaining piece which is interposed between the adjacentballs of a row of rolling balls, and concave surfaces of both sidesurfaces of the retaining piece as viewed in the axial direction are incontact with the spherical surfaces of the balls. The retaining piecestructure is characterized in that each the concave surface isconfigured such that as the wearing and/or yielding of the contactportion of the concave surface with the ball progress, the concavesurface increases a contact area thereof with the ball.

According to a third aspect of the invention, there is provided astructure of a retaining piece which is interposed between the adjacentballs of a row of rolling balls, and concave surfaces of both sidesurfaces of the retaining piece as viewed in the axial direction are incontact with the spherical surfaces of the balls. The retaining piecestructure is characterized in that each the concave surface includes alinear contact portion to be in contact with the ball, which is locatedclose to the outer peripheral edge thereof, and an approximate curvedsurface having the radius of curvature approximate to that of the ball,which is located on the inner side of the linear contact portion (claim3).

The invention also provides a linear motion device provided withretaining pieces each having any of the retaining piece structuresdescribed above.

In the retaining piece structure of the invention, each of the concavesurfaces of the retaining piece a linear contact portion to be incontact with the ball, and an approximate curved surface having theradius of curvature approximate to that of the ball. Normally, the balllinearly contacts with the retaining piece, while the approximate curvedsurface having the radius of curvature approximate to that of the balldoes not contact with the ball with a slight gap therebetween.

When a load acting on the retaining piece increases to excess a loadvalue, or when the concave surface of the retaining piece is worn and/oryielded, a preset gap quantity between the retaining piece and the ballis reduced, and a span between the adjacent balls is reduced. Then, theball starts to contact with the approximate curved surface having theradius of curvature approximate to that of the ball in the concavesurface of the retaining piece. As a result, a contact area of the ball3 with the approximate curved surface 102 increases, and no furtherreduction of the span L progresses.

Next, to achieve the above second object, a fourth to ninth aspect ofthe invention are provided as follows. According to a fourth aspect ofthe invention, there is provided a structure of a retaining piece whichis interposed between the adjacent balls of a row of rolling balls, andconcave surfaces of both side surfaces of the retaining piece as viewedin the axial direction are in contact with the spherical surfaces of theballs, the improvement comprising a gate portion used in a process ofmolding retaining pieces is provided in the concave surface.

According to a fifth aspect of the invention, in the retaining piecestructure of the fourth aspect of the invention, a recess is provided ata joining portion joined to the gate portion used in the process ofmolding the retaining pieces.

According to a sixth aspect of the invention, there is provided astructure of a retaining piece which is interposed between the adjacentballs of a row of rolling balls, and concave surfaces of both sidesurfaces of the retaining piece as viewed in the axial direction are incontact with the spherical surfaces of the balls, the improvementcomprising a holder portion which holds an ejector pin used in a processof molding the retaining pieces is positioned in the concave surface.

According to a seventh aspect of the invention, in the retaining piecestructure, a recess is provided at a joining portion joined to theejector pin used in the process of molding the retaining pieces.

According to an eighth aspect of the invention, there is provided astructure of a retaining piece which is interposed between the adjacentballs of a row of rolling balls, and concave surfaces of both sidesurfaces of the retaining piece as viewed in the axial direction are incontact with the spherical surfaces of the balls, the improvementcomprising a parting line used in a process of molding the retainingpieces is set on the outer peripheral surface of the retaining piece,and a recess or a tapered portion is provided near the parting line onthe outer peripheral surface.

According to a ninth aspect of the invention, there is provided a linearmotion device provided with retaining pieces each having any of theretaining piece structures described above.

In the retaining piece structure of the invention, the gate portion andthe ejector pin, which are used in the process of molding the retainingpieces, are provided in the concave surfaces of both side surfaces ofthe retaining piece as viewed in the axial direction. Thus, in the moldseparation step after the molding process, the retaining piece isautomatically separated from the gate. Therefore, the gate cuttingprocess, which is essential to the conventional technique, iseliminated, and this leads to cost reduction.

A recess is provided at a joining portion of the concave surface to thegate portion or the ejector pin position or near the parting line. Inaddition, fins and the like formed in the molding process are out in therecess. Accordingly, it never happens that when the retaining pieces areincorporated into the linear motion device, the fins or the like come incontact with the balls of the linear motion device and the inside andoutside guide members, to thereby deteriorate the operability and thenoise characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing an embodiment of a retaining piecestructure according to the present invention;

FIG. 2 is a cross sectional view taken line II—II in FIG. 1;

FIG. 3 is a front view showing a second embodiment of a retaining piecestructure according to the present invention;

FIG. 4 is a cross sectional view taken line IV—IV in FIG. 3;

FIG. 5 is a front view showing a third embodiment of a retaining piecestructure according to the present invention;

FIG. 6 is a cross sectional view taken line VI—VI in FIG. 5;

FIG. 7 is a cross sectional view showing a retaining piece structurewhich is a fourth embodiment of the present invention.

FIG. 8 is a cross sectional view showing a retaining piece structurerelated to the invention.

FIG. 9 a perspective view, partly cut out, showing a key portion of alinear guide as a linear motion device into which the retaining piecestructure of the invention may be applied;

FIG. 10 is a perspective view, partly cut out, showing a key portion ofa ball screw mechanism as a linear motion device into which theretaining piece structure of the invention may be applied; and

FIG. 11 is a diagram showing a model of a retaining piece structurerelated to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 is a front view showing an embodiment of a retaining piecestructure according to the present invention. FIG. 2 is a crosssectional view taken line II—II in FIG. 1. Construction descriptionabout the invention will first be given. A retaining piece 100 isinterposed between the balls B in the ball endless circulation track 9of the linear guide as the linear motion device (FIG. 9) or the ballcirculation track 18 of the ball screw (FIG. 10). The construction ofthe linear motion device per se is the same as of the already describedone except the retaining pieces 100. Hence, no further description of itwill be given here.

The retaining piece 100 is made of a material excellent in the injectionmolding and wear proof, such as 66 nylon or whiskers-contained 66 nylon.It takes the form of a short cylindrical member smaller in diameter thanthe steel ball 8. Both sides of the retaining piece 100 are inwardcurved at the radius of curvature, which is approximate to the radius RBof the steel ball B, into concave surfaces 101. The details of theconcave surfaces 101 will be described hereunder.

A central portion of the concave surface 101 is an approximate curvedsurface (spherical surface) 102 whose radius of curvature is approximateto the curvature radius RB of the ball B. The concave surface 101further includes a ring-like linear contact portion 103 where itlinearly contacts with the ball B. The linear contact portion 103 islocated closer to the outer peripheral edge of the concave surface 101,while surrounding the approximate curved surface 102. A circular brokenline depicted within the linear contact portion 103 indicates a phantomlinear contact line 103 a where it contacts with the ball B. A positionof the phantom linear contact line is displaced within a width range ofthe linear contact portion 103 in accordance with a variation of apressing force of the ball B. In other words, the concave surface 101 isconfigured such that so long as a variation of the pressing force of theball B is within a predetermined range of force values, its linearcontact with the ball B is maintained, and when the pressing force ofthe ball B increases to be in excess of a force value within thepredetermined range, the ball B begins to contact with the outerperipheral portion of the approximate curved surface 102 and its contactarea with the ball B gradually increases.

An operation of the retaining piece 100 thus structured will bedescribed.

In a normal state that the pressing force of the ball B is not large,the ball B linearly contacts with a part of the linear contact portion103, and a small gap of about 30 to 50 μm, as shown in FIG. 2, ispresent between the approximate curved surface 102 and the outer surfaceof the ball B. When comparing with the conventional retaining piecestructure in which the concave surface entirely contacts with the ball Bfrom the first, sliding resistance between the retaining piece and theball is small, and hence low noise generation and high operability areobtained.

When a load acting on the retaining piece 100 increases to excess a loadvalue within which the linear contact is maintained, or when the concavesurface 101 of the retaining piece is worn and/or yielded as the resultof long time use, a preset gap quantity C between the retaining piece100 and the ball is reduced, and a span L between the adjacent balls isreduced. Accordingly, the ball B starts to contact with the approximatecurved surface 102 of the concave surface of the retaining piece, acontact area of the ball B with the approximate curved surface 102increases, and no further reduction of the span L progresses. Thus, avariation of the ball-to-ball span L is reduced while a proper contactarea is maintained. Therefore, the good operability and the improvednoise characteristics are kept.

In consideration of a quantity of the overlapping portion of the ball Band the like, in the embodiment, the concave surface 101 is configuredso as to be most suitable for the operability improvement: “The linearcontact portion 103 is located closer to the outer peripheral edge ofthe concave surface 101 and the approximate curved surface 102 islocated in the central portion of the same.” If required, the concavesurface 101 may be configured as follows: “The linear contact portion103 is located in the central portion of the concave surface 101 and theapproximate curved surface 102 is located close to the outer peripheraledge.” In the latter case, the overlapping portion of the ball B isreduced, so that the operability is somewhat inferior to that of theformer.

With regard to the increase of the contact area, the concave surface isnot limited to the one configured such that the contact area abruptlyincreases as in the embodiment, but it may be a concave surfaceconfigured such that the contact area gradually increases, that is, acurved surface whose curvature radius is approximate to that of the ballmay be reshaped into a curved surface whose curvature radius is somewhatsmaller than that of the ball.

The concave surface may be configured so as to be the most suitable foruse conditions in consideration of the balance of the operability (thebetter it is, the smaller the contact area is) and the durability (it isbetter when the ball-to-ball span L is invariable and as the contactarea increases).

FIGS. 3 and 4 show a second embodiment of a retaining piece structureaccording to the present invention. The retaining piece 100A of thisembodiment is different from that of the first embodiment in that slitportions 110 are formed in the concave surface 101 thereof. Since theslit portions 110 serve as lubricant reservoirs. Accordingly, theoperability and the durability will be improved. Foreign matters alsomove into and are stored in the slit portions 110, so that the retainingpiece 100A is less worn. For this reason, in particular under hardcircumstances where foreign matters and the like are present, thedurability in the second embodiment is superior to that of the firstembodiment.

The other construction, operation and effects of the second embodimentare substantially the same as those of the first embodiment.

FIGS. 5 and 6 show a third embodiment of a retaining piece structureaccording to the present invention.

The retaining piece 100B of this embodiment is different from thoseretaining pieces of the first and second embodiments in that a throughhole 120, which axially extends, is provided at the central portion ofthe concave surface 101. The through hole 120 serves as a lubricantreservoir, so that the operability and the durability will be bothimproved. Foreign matters also move to and put into the through hole120. As a result, the wearing of the retaining piece 100A is lessened.In particular under hard use circumstances where foreign matters and thelike are present, this embodiment is superior in durability to the firstembodiment. The other construction, operation and effects of thisembodiment are similar to those of the first embodiment.

FIG. 7 shows a fourth embodiment of a retaining piece structureaccording to the present invention.

In the retaining piece 100C of this embodiment, the retaining piece 100Cis made of a material excellent in the injection molding, wear proof,rigidity and the like, such as 66 nylon or whiskers-contained 66 nylon.It takes the form of a short cylindrical member smaller in diameter thanthe steel ball B. If known lubricant is contained in the material, theoperability and durability are both improved. Both sides of the shortcylindrical member are inward curved at the radius of curvature, whichis approximate to the radius RB of the steel ball B, into concavesurfaces 201. The details of the concave surfaces 201 will be describedhereunder. The curvature center 0 ₁₁₁ and 0 ₂₁₁ of the concave surfaces201 lie on an axial line J. Accordingly, the thickness of the retainingpiece, which is located between the concave surfaces 201, is minimum invalue at the bottom position of the concave surfaces through which theaxial line J passes.

A relatively shallow recess 202 p is formed in the bottom portion of oneof the concave surfaces 201, while a relatively deep recess 202G isformed in the bottom portion of the other concave surface 201. Theshallow recess 202 p serves as a portion to which a ejector pin P(indicated by a phantom line in FIG. 7) of a mold in the process ofinjection molding the retaining piece 100C, viz., a pin joining portion.The deep recess 202G is a joining portion for a gate G (indicated by aphantom line in FIG. 7) of a mold in the process of injection moldingthe retaining piece 100C. The above-mentioned thickness “t” of theretaining piece 100C is selected to be such a thickness as to allow theformation of the shallow recess 102 p and the deep recess 202G.Therefore, the load capacity and rigidity of the linear motion devicewhich are as high in level as possible, are secured.

In the embodiment, a parting line PL is provided on an outer peripheralsurface 203 as the outer peripheral surface of the cylindrical member. Arecess (or may be a tapered portion) 204 is provided also near and alongthe parting line PL, while entirely surrounding the outer peripheralsurface 203.

The operation of the retaining piece 100C will be described.

In the case of the retaining piece 100C, the portions to be joined tothe ejector pin P and the gate G in the molding process are the bottomsurfaces of the shallow recess 202 p and the deep recess 202G.Therefore, in separating the product from the mold, the retaining piece100C is automatically separated from the gate G. Accordingly, the gatecutting process, which is essential to the conventional technique, iseliminated, and this leads to cost reduction.

The portion to be butted with the ejector pin P for ejecting the productout of the mold, and gate remainder after the gate port is cut out,viz., called fins, are put in the shallow recess 202 p and the deeprecess 202G. Therefore, there is no case that those fins are protrudedas obstacle things from the concave surfaces 201. Accordingly, it neverhappens that when the retaining pieces 100C are incorporated into thelinear motion device, such as a linear guide or a ball screw mechanism,the fins or the like come in contact with the balls in the linear motiondevice, the inside guide members (e.g., the return guide and the slider)and the outside guide members (e.g., the end caps and the guide rail) ofthe ball circulation track, to thereby deteriorate the operability andthe noise characteristic.

In the retaining piece 100C, the recess 204 is provided near the partingline PL set on the outer peripheral surface 203. Therefore, protrusionof the parting line PL formed in the molding process is put into therecess 204, and is not protruded outside. In this respect, the inventionsucceeds in solving the problem of the conventional technique that whenthe retaining piece 100C is incorporated into the linear motion device,the protrusion of the parting line comes in contact with the inside andoutside guide members of the linear motion device to deteriorate theoperability and the noise characteristic.

Thus, the invention provides a retaining piece structure which is low innoise and high in operability, and minimizes the reduction of the loadcapacity and rigidity. The other construction, operation and effects ofthis embodiment are similar to those of the first embodiment.

The linear motion device using the retaining piece structure of theinvention is not limited to the linear guide, but may be applied toother linear motion devices, such as a ball screw mechanism. In short,the invention may be applied to a device for causing a linear motion bythe utilization of rolling of the row of rolling elements. By so doing,the improvement of the operability and the noise characteristic isobtained.

While the retaining pieces used in the above-mentioned embodiments areseparated from one another, it is readily understood that thoseretaining pieces coupled to one another may be used.

The invention has the following effects by the structure. As seen fromthe foregoing description, as the load acting on the concave surfaces ofthe retaining pieces each being interposed between the adjacent rollingpieces increases, or as the wearing and yielding of the concave surfaceof each retaining piece progress, the contact areas of the retainingpiece to the rolling elements are increased, thereby reducing avariation of the element-to-element span. Therefore, the presentinvention is capable of maintaining good operability and good noisecharacteristic of the linear motion device for a long time even undervarious use conditions.

In addition, as seen from the foregoing description, the thickness ofthe retaining piece interposed between the rolling elements isminimized, and the recesses are formed in the concave surfaces of theretaining piece, which are to be in contact with the balls, and/or theouter peripheral surface thereof. Therefore, there is no case where theoperability and the noise characteristic are deteriorated by harmfulfins. Accordingly, the operability and the noise characteristic of thelinear motion device are improved, and the reduction of the loadcapacity and rigidity is lessened.

Further, when each structure or the above-mentioned embodiments are usedtogether, the operability and the noise characteristic of the linearmotion device are remarkably improved, and the reduction of the loadcapacity and rigidity is also lessened.

What is claimed is:
 1. A structure of a retaining piece which isinterposed between each two adjacent rolling elements in a row, thestructure comprising concave surfaces at both side surfaces of saidretaining piece in an axial direction of said row, each of said concavesurfaces being in contact with a spherical surface of a adjacent rollingelement, wherein each of said concave surface comprises a holder portionwhich holds an ejector pin used in a process of molding said retainingpiece.
 2. The retaining piece structure according to claim 1, whereineach of said concave surfaces comprises: a jointing portion jointed tosaid ejector pin; and a recess portion provided at said joining portion.3. A linear motion device comprising the retaining piece according toclaim
 1. 4. A structure of a retaining piece which is interposed betweeneach two adjacent rolling elements in a row, the structure comprisingconcave surfaces at both side surfaces of said retaining piece in anaxial direction of said row, each of said concave surfaces being incontact with a spherical surface of an adjacent rolling element, saidconcave surface being configured such that as a pressing force actingthereon increases, each said concave surface increases a contact areathereof with said adjacent rolling element; and wherein each of saidconcave surfaces comprises a holder portion which holds an ejector pinused in a process of molding said retaining piece.
 5. The structure ofthe retaining piece according to claim 4, wherein each of said concavesurface comprises: jointing portion jointed to said ejector pin; and arecess portion provided at said joining portion.
 6. A linear motiondevice comprising the retaining piece according to claim 2.